|Desired traits for the biofuels-optimized poplar. Click to enlarge. Source: DOE|
A four-year scientific and technical effort has resulted in the sequencing and publication of the genome of the poplar tree. The effort, led by the US Department of Energy’s Joint Genome Institute (DOE JGI) and Oak Ridge National Laboratory (ORNL), united the efforts of 34 institutions from around the world, including the University of British Columbia, and Genome Canada; Umeå Plant Science Centre, Sweden; and Ghent University, Belgium.
A paper highlighting the analysis of this first complete DNA sequence of a tree is published in this week’s edition of the journal Science. The work on the Populus trichocarpa—black cottonwood—genome will support the development of trees as a ideal feedstock for a new generation of biofuels such as cellulosic ethanol.
Poplar’s extraordinarily rapid growth and its relatively compact genome size—480 million nucleotide units, 40 times smaller than the genome of pine—are among the many features that led researchers to target poplar as a model crop for biofuels production.
In the DOE-USDA Billion-Ton Study, corn stover and perennial crops such as switchgrass and hybrid poplar make up about half the potential 1.3 billion tons of annual biomass that could be available by the mid-21st Century. (Earlier post.)
(The Billion-Ton Study found that the United States has enough agricultural and forestry land to support production of more than one billion tons of biomass annually, which could provide enough liquid biofuels to replace more than a third of current transportation fuel consumption, and still continue to meet food, feed, and export demands.)
Under optimal conditions, poplars can add a dozen feet of growth each year and reach maturity in as few as four years, permitting selective breeding for large-scale sustainable plantation forestry. This rapid growth coupled with conversion of the lignocellulosic portion of the plant to ethanol has the potential to provide a renewable energy resource along with a reduction of greenhouse gases.—Dr. Sam Foster, US Forest Service
Among the major discoveries yielded from the poplar project is the identification of more than 45,000 protein-coding genes, more than any other organism sequenced to date, approximately twice as many as present in the human genome (which has a genome six times larger than the poplar’s).
The research team identified 93 genes associated with the production of cellulose, hemicellulose and lignin, the building blocks of plant cell walls. The biopolymers cellulose and hemicellulose constitute the most abundant organic materials on earth, which by enzymatic action, can be broken down into sugars that in turn can be fermented into alcohol and distilled to yield fuel-quality ethanol and other liquid fuels.
Poplar is the most complex genome to be sequenced and assembled by a single public sequencing facility and only the third plant to date to have its genome completely sequenced and published.
In a research roadmap for cellulosic ethanol—Breaking the Biological Barriers to Cellulosic Ethanol: A Joint Research Agenda—released earlier this summer (earlier post), the DOE outlines some of the traits of a poplar optimized for biofuels.
Fundamentally, the goal is deliver faster-growing trees that produce more readily-convertible biomass. For example, genetic engineering has already been used to limit and alter lignification to maximize biomass-to-energy conversion. The resulting trees had normal growth and development, but the pulping time was reduced by more than 60%.
In an editorial in Science earlier in the year, Steven Koonin, Chief Scientist for BP, wrote:
Genetic improvement of energy crops such as switchgrass, poplar, and jatropha has barely begun. It will be important to increase the yield and environmental range of energy crops while reducing agricultural inputs. Plant development, chemical composition, tolerance of biotic and abiotic stresses, and nutrient requirements are important traits to be manipulated. The combination of modern breeding and transgenic techniques should result in achievements greater than those of the Green Revolution in food crops, and in far less time.
The poplar project supports a broader DOE drive to accelerate research into biofuels production. In August, the department announced it would spend $250 million over five years to establish and operate two new Bioenergy Research Centers. (Earlier post.) The DOE-supported research into biofuels is focusing on both plants and microbes, in an effort to discover new biotechnology-based methods of producing fuels from plant matter (biomass) cost-effectively.
Secretary of Energy Samuel W. Bodman has set a departmental goal of replacing 30% of current transportation fuel demand with biofuels by 2030.
|Geographic distribution of potential biomass energy crops. Popular (outlined in yellow) can grow in most targeted regions of the country. Click to enlarge. Source: DOE.|
Vast poplar farms in regions such as the Pacific Northwest, the upper Midwest, and portions of the southeastern U.S. could provide a steady supply of tree biomass rich in cellulose that can be transformed by specialized biorefineries into fuels like ethanol. Other regions of the country might specialize in different energy crops suited to their particular climate and soil conditions, including such plants as switchgrass and willow. In addition, a large quantity of biofuels might be produced from agricultural and forestry waste.
The DOE Joint Genome Institute, supported by the DOE Office of Science, unites the expertise of five national laboratories, Lawrence Berkeley, Lawrence Livermore, Los Alamos, Oak Ridge, and Pacific Northwest, along with the Stanford Human Genome Center to advance genomics in support of the DOE mission related to clean energy generation and environmental characterization and clean-up.
DOE JGI’s Walnut Creek, Calif. Production Genomics Facility provides integrated high-throughput sequencing and computational analysis that enable systems-based scientific approaches to these challenges.
“The Genome of Black Cottonwood, Populus trichocarpa (Torr. & Gray)”; G. A. Tuskan et. al.; Science 15 September 2006: Vol. 313. no. 5793, pp. 1596 - 1604; DOI: 10.1126/science.1128691
“Getting Serious About Biofuels”; Koonin, S. E.; Science 311, 435, 2006